1. Field of the Invention
This invention is related to the field of probe-based detection, analysis and/or quantitation of microorganisms. More specifically, this invention relates to novel PNA probes, probe sets, methods and kits pertaining for the detection, identification and/or enumeration of organisms of the various species of the Candida genus.
2. Description of the Related Art
Nucleic acid hybridization is a fundamental process in molecular biology. Probe-based assays are useful in the detection, quantitation and/or analysis of nucleic acids. Nucleic acid probes have long been used to analyze samples for the presence of nucleic acid from bacteria, fungi, virus or other organisms and are also useful in examining genetically-based disease states or clinical conditions of interest. Nonetheless, probe-based assays have been slow to achieve commercial success. This lack of commercial success is, at least partially, the result of difficulties associated with specificity, sensitivity and reliability.
Despite its name, Peptide Nucleic Acid (PNA) is neither a peptide, a nucleic acid nor is it an acid. Peptide Nucleic Acid (PNA) is a non-naturally occurring polyimide that can hybridize to nucleic acid (DNA and RNA) with sequence specificity (See: U.S. Pat. No. 5,539,082 and Egholm et al., Nature 365: 566-568 (1993)). Being a non-naturally occurring molecule, unmodified PNA is not known to be a substrate for the enzymes that are known to degrade peptides or nucleic acids. Therefore, PNA should be stable in biological samples, as well as have a long shelf-life. Unlike nucleic acid hybridization, which is very dependent on ionic strength, the hybridization of a PNA with a nucleic acid is fairly independent of ionic strength and is favored at low ionic strength, conditions that strongly disfavor the hybridization of nucleic acid to nucleic acid (Egholm et al., Nature, at p. 567). The effect of ionic strength on the stability and conformation of PNA complexes has been extensively investigated (Tomac et al., J. Am. Chem. Soc. 118:55 44-5552 (1996)). Sequence discrimination is more efficient for PNA recognizing DNA than for DNA recognizing DNA (Egholm et al., Nature, at p. 566). However, the advantages in point mutation discrimination with PNA probes, as compared with DNA probes, in a hybridization assay, appears to be somewhat sequence dependent (Nielsen et al., Anti-Cancer Drug Design 8:53-65, (1993) and Weiler et al., Nucl. Acids Res. 25: 2792-2799 (1997)).
Though they hybridize to nucleic acid with sequence specificity (See: Egholm et al., Nature, at p. 567), PNAs have been slow to achieve commercial success at least partially due to cost, sequence specific properties/problems associated with solubility and self-aggregation (See: Bergman, F., Bannwarth, W. and Tam, S., Tett. Lett. 36:6823-6826 (1995), Haaima, G., Lohse, A., Buchardt, O. and Nielsen, P. E., Angew. Chem. Int. Ed. Engl. 35:1939-1942 (1996) and Lesnik, E., Hassman, F., Barbeau, J., Teng, K. and Weiler, K., Nucleosides & Nucleotides 16:1775-1779 (1997) at p 433, col. 1, ln. 28 through col. 2, ln. 3) as well as the uncertainty pertaining to non-specific interactions that might occur in complex systems such as a cell (See: Good, L. et al., Antisense & Nucleic Acid Drug Development 7:431-437 (1997)). However, problems associated with solubility and self-aggregation have been reduced or eliminated (See: Gildea et al., Tett. Lett, 39: 7255-7258 (1998)). Nevertheless, their unique properties clearly demonstrate that PNA is not the equivalent of a nucleic acid in either structure or function. Consequently, PNA probes should be evaluated for performance and optimization to thereby confirm whether they can be used to specifically and reliably detect a particular nucleic acid target sequence, particularly when the target sequence exists in a complex sample such as a cell, tissue or organism.